This invention relates generally to feedthrough capacitor terminal pin subassemblies and related methods of construction, particularly of the type used in implantable medical devices such as cardiac pacemakers, implantable defibrillators, cochlear implants, and the like. Such terminal pin subassemblies form EMI filters designed to decouple and shield undesirable electromagnetic interference (EMI) signals from an associated device. Specifically, the present invention relates to an improved EMI filter that includes an inductive element, making the EMI filter a two element (2-pole) or three element (3-pole) device, or even higher order device. Feedthrough terminal assemblies are generally well known for connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices, such as cardiac pacemakers, defibrillators, or the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. In a cardiac pacemaker, for example, the feedthrough terminal pins are typically connected to one or more lead wires within the case to conduct pacing pulses to cardiac tissue and/or detect or sense cardiac rhythms.
However, the lead wires can also effectively act as an antenna and thus tend to collect stray electromagnetic interference (EMI) signals for transmission into the interior of the medical device. Studies conducted by the United States Food and Drug Administration, Mt. Sinai Medical Center in Miami and other researchers have demonstrated that stray EMI, such as that caused by cellular phones, can seriously disrupt the proper operation of the pacemaker. It has been well documented that pacemaker inhibition, asynchronous pacing and missed beats can occur. All of these situations can be dangerous or life threatening for a pacemaker-dependant patient.
In prior devices, such as those shown in U.S. Pat. Nos. 5,333,095 and 4,424,551 (the, contents of which are incorporated herein), the hermetic terminal pin subassembly has been combined in various ways with a ceramic feedthrough capacitor filter to decouple electromagnetic interference (EMI) signals into the housing of the medical device. FIG. 1 is a cross-sectional view of the feedthrough terminal assembly disclosed is U.S. Pat. No. 5,333,095. Within the drawings herein, functionally equivalent elements of structure shown in the drawings will be referred to by the same reference number irrespective of the embodiment shown. The assembly 10 includes a conductive ferrule 12 which is conductively connected to a housing or casing 14 of a human implantable device, such as a cardiac pacemaker, an implantable defibrillator, or a cochlear implant or the like. The assembly 10 includes a feedthrough capacitor 16 having a grounding portion 24 which is conductively coupled to the ferrule 12. At least one terminal pin or lead wire 18 extends through the ferrule 12, in non-conductive relation, and through the capacitor 16 in conductive relation. Typically, an alumina insulator 20 is disposed between the terminal pin 18 and the ferrule 12 or other conductive substrate through which the terminal pin 18 passes through in non-conductive relation. The capacitor 16 may be bonded to the insulator 20 or separated from the insulator 20 thereby forming an air gap depending on the assembly method used. Typically, the outside diameter metallization 24 of the capacitor 16 is installed in conductive relation with the conductive substrate or ferrule 12 so that the ground electrodes of feedthrough capacitor 16 are properly grounded. An alternative arrangement is shown in U.S. Pat. No. 5,905,627, the contents of which are incorporated herein.
FIG. 2 illustrates the uni-polar monolithic ceramic feedthrough capacitor 16 of FIG. 1, which is typical in the prior art described by the U.S. Pat. Nos. 5,333,095 and 4,424,551 patents and many others. Both inside diameter and outside diameters 22 and 24 are metallized using a conductive termination which puts the respective electrode plate sets in parallel. The feedthrough capacitor is designed to have the lead wire 18 pass through the center of it. The lead wire or terminal pin 18 is conductively coupled to the inner diameter metallization 22 so as to be conductively coupled to a first set of active electrodes 26. A second set of ground electrodes 28 are conductively coupled to the outer diameter metallization 24 for grounding to the conductive substrate or ferrule 12.
FIG. 3 is the schematic diagram of the feedthrough capacitor of FIG. 2. As shown, feedthrough capacitors are three terminal devices which offer broadband performance and are best modeled by transmission line equations. Feedthrough capacitors are novel in that they act like broadband transmission lines and have very low inductance properties. This means that they can provide effective EMI filtering immunity over very broad frequency ranges. They do this by de-coupling high frequency noise and shunting it to the overall titanium or stainless shield housing 14 of the implantable medical device. This is in contrast to rectangular monolithic chip capacitors and other two terminal capacitors which have a substantial amount of series inductance. Two terminal capacitors tend to self resonate at very low frequency and thus make very poor EMI filters, particularly for high frequencies such as cell phones, microwave ovens, radars and other emitters.
FIGS. 4 and 5 illustrate another type of capacitor 16, which is a multi-hole micro-planar array quad-polar feedthrough capacitor. This has essentially the same properties as the previously described uni-polar feedthrough capacitor illustrated in FIGS. 2 and 3, and can accommodate multiple terminal pins therethrough. FIG. 6 is the schematic drawing of the quad-polar capacitor of FIGS. 4 and 5.
FIG. 7 describes the capacitor reactance equation and illustrates how the capacitor reactance varies in ohms vs. frequency for an ideal capacitor. At DC, capacitors look like open circuits (in other words, like they are not there). At high frequencies, well-designed capacitors tend to look like a very low reactance in ohms (or short circuit). In this way, capacitors are frequency selective components and can be used to short out or bypass undesirable high frequencies thereby acting as low pass filter devices.
In the past few years, a number of new devices have been introduced to the active implantable medical device market. These include implantable cardioverter defibrillators, which not only offer high voltage shock therapy to the heart, but also provide monitoring, anti-tachycardia pacing and conventional atrial and ventricular pacing. Very recently introduced are congestive heart failure devices, also known on the market as biventricular pacemakers. All of these new devices have a need for an increased number of lead wires to be implanted within the heart or outside the vasculature of the heart. This has greatly complicated the loop coupling and antennae coupling areas for EMI induction. This also means that more lead wires must ingress and egress the implantable medical device. Accordingly, it is now common for 8-pin, 12-pin or even 16-pin devices to be present in the marketplace, all of which have unique filtering needs.
There have also been new developments in sensor technology. Lead based sensors are under investigation as well as new telemetry methods. The Federal Communications Commission has recently opened up higher frequency telemetry channels (402 MHz) to meet the demands for more bandwidth on the part of physicians (better access to stored data, recovery of historical cardiac waveforms, etc.). Most modern pacemakers and implantable defibrillators store a substantial amount of data and can download cardiac waveforms for later investigation by the physician.
There has also been an increase in the number of emitters generally in the marketplace. An example of this is the new Blue Tooth System, which is rapidly gaining acceptance. Blue Tooth is a method of interconnecting computers and the peripheral devices in a wireless manner. This also increases the number of digital signals to which an implantable device patient is exposed. Accordingly, there is an ever-increasing need for better EMI immunity of implantable medical devices over wider frequency ranges.
As mentioned, there has been a substantial amount of research into the interaction of implantable medical devices with cellular phones, theft detectors and other emitters. This research is ongoing today, particularly in the area of cardiac pacemakers and ICDs. Recently, high-gain cellular telephone amplifiers combined with high-gain antennas have become available in consumer markets. This creates a concern because the single element EMI filters presently designed into pacemakers and ICDs are based on research when cellular telephone maximum output power was limited to 0.3 or 0.6 watts. When a cellular phone is combined with these new amplifiers and high-gain antennas, the output power increases by a factor of 20 to 30 dB. This is equivalent to a 23.8-watt cell phone.
Prior art EMI filters for medical implant applications have generally consisted of single pole devices consisting of a single feedthrough capacitor element on each lead wire. It is possible to increase the amount of attenuation of a single element feedthrough capacitor by raising the capacitance value. This also desirably lowers the frequency at which the capacitor starts to become effective. This is known as the feedthrough capacitor's 3 dB cutoff point. Unfortunately, raising the capacitance also has a number of undesirable side effects. First of all, too much capacitance can start loading down the output of an implantable medical device thereby degrading its operation. Too much capacitance can also be a problem in that excess energy dissipation can occur as the capacitor must be charged and discharged during cardiac pacing or digital signal processing in a hearing device.
In an EMI filter design of a low pass filter, a single element filter consisting of a feedthrough capacitor increases in attenuation at 20 dB per decade. This is a consequence of the mathematics of computing the capacitive reactance as described in FIG. 7 and its behavior as a low pass filter circuit. The capacitive reactance Xc in ohms varies inversely as the capacitance value and also inversely with frequency.
An inductor performs the opposite function in that the inductive reactance XL in ohms, as shown in FIG. 8, varies directly with the frequency and the inductance in microhenries. This formula is applicable not only to multi-turn toroids, but single turn ferrite beads as well. The inductive reactance XL is the opposite of capacitance reactance Xc in that inductive reactance increases with increasing frequency. As illustrated, inductive reactance is zero ohms at DC and goes up to a very high value at high frequency.
Therefore, when placed in series with a line, inductance can raise the impedance of the line thereby also acting as a low pass filter. Common prior art EMI filter circuits are shown in FIG. 9 consisting of single element feedthrough capacitors “C”, “double element L1” and “reverse L2” filters, which combine an inductor and a capacitor, and other elements or other configurations including “PI” and “T” configurations. The commonly used prior art filter circuit for medical implant applications has been the “C” circuit or feedthrough capacitor. All of the cited patent references are based on a single element feedthrough capacitors bonded directly to or in close proximity to the hermetic terminal of an implantable medical device. However, using inductance in combination with a feedthrough capacitor increases the filter's effectiveness.
Of particular interest are the graphs shown in FIG. 10. The horizontal or X axis is frequency in MHz and the vertical or Y axis is the filtering efficiency measured as insertion loss in dB. For a one component feedthrough capacitor filter “C”, the insertion loss increases with frequency at a slope of 20 dB per decade. However, when one adds an inductive component this makes the low pass filter into a two-element “L” filter. A two element filter like an “L” filter goes up at a slope of 40 dB per decade. This means that its filtering effectiveness at high frequency is much greater than a single element filter. If one were to add inductors on both sides of the capacitor, it would become a three component filter, which would increase at 60 dB per decade and so on.
A single element feedthrough capacitor is limited to an attenuation increase of 20 dB per decade. This is a linear function on semi log paper in the region that is well above the 3 dB cutoff point. In other words, for a single element feedthrough capacitor filter that offers 20 dB of attenuation at 10 MHz, that same filter would offer 40 dB at 100 MHz which is one frequency decade above. If one were to take the same feedthrough capacitor and combine with it an inductor element, thereby making it into an L section filter, this now becomes a 2-element filter. A 2-element filter will increase its attenuation effectivity by 40 dB per decade. Using the example as previously illustrated, if an L section filter, which is well above cutoff, exhibits 20 dB of attenuation at 10 MHz, it will exhibit 60 dB of attenuation at 100 MHz which is a very dramatic increase in filtering effectivity.
This is uniquely advantageous in an implantable medical device in that one can greatly increase the amount of attenuation of the EMI filter in frequency ranges at 1 MHz and above where many problem emitters transmit. For example, in the 22 and 72 MHz frequency ranges, hand held or chest strap transmitters are commonly used to control model airplanes, model helicopters and remote control boats. These sophisticated devices produce powerful digitally controlled signals which can be in very close proximity to an implanted medical device. Accordingly, a two element EMI filter can be designed such that it offers very low attenuation in the cardiac sensing and telemetry ranges of the implantable medical device, but increases the attenuation curve very steeply above these frequencies. Accordingly, there is a need to provide multi-element filters for implantable medical devices.
As described herein, adding inductance in series with pacemaker or implantable defibrillator leads is dramatically effective. It has been found that the input impedance ZIN in pacemaker biological signal sensing circuits is relatively high at low frequencies (ZIN above 10,000 ohms) but can be quite low and, parasitically variable at high frequencies (ZIN well below 5). It is a novel feature of the present invention that the addition of inductive element to the feedthrough capacitor raises and stabilizes the input impedance of the active implantable medical device (AIMD), particularly at these certain parasitic frequencies. In a two element “L” filter, it is important that the inductor element be placed on the side of the capacitor toward the internal electronic circuitry of the AIMD. By thereby raising and stabilizing the AIMD input impedance, the feedthrough capacitor, which is oriented toward the body fluid side, first intercepts and thereby becomes much more effective in bypassing high frequency EMI signals to the overall equipotential shield or housing of the AIMD. This shunting of undesirable signals prevents EMI signals from entering into the AIMD housing where they could interfere with proper AIMD circuit and therapy functions.
Exemplary ferrite beads and wire-wound inductors 30–34 are illustrated in FIGS. 11–15. FIG. 15 illustrates placing multiple turns of wire 36 through a ferrite or iron-core inductor element 34. This is highly efficient because the inductance of the component goes up as the square of the number of turns. In other words, if one were to place a single turn or a straight lead wire 36 through the ferrite bead element or ferrite core 32, this would be defined as one turn (FIGS. 13 and 14). However, if one were to place additional turns, the inductance would go up as the square of the number of turns. FIG. 15 illustrates a three-turn inductor as counted by three passes of the wire 36 through the center hole of the toroidal inductor core 34. This would have 9 times the inductance of the device as shown in FIG. 13, which has one pass of wire 36 through the center hole. The toroidal inductor material can be made of ferrite, powdered iron, molypermalloy or various other materials which affect inductive properties.
Another major trend affecting active implantable medical devices is the ever-increasing need for smaller size devices. Just a few years ago, implantable cardioverter defibrillators (ICD's) were over 100 cubic centimeters in volume. Today, ICDs are being designed below 30 cubic centimeters. Thus, the size of all components within the active medical device must be as small as possible. Therefore, it is not practical to add inductive or ferrite elements if they are to take up additional space inside the implantable medical device.
Typical values for filter feedthrough capacitors used in medical implant applications range from 390 picofarads all the way up to 9000 picofarads. The average feedthrough capacitor, however, is not very volumetrically efficient. Since only a few electrode plates are required to reach the desired capacitance value (due to the high dielectric constant), typical feedthrough capacitors used in medical implantable devices incorporate a number of blank cover sheets. A typical ceramic feedthrough capacitor used in an active implantable medical device would have a thickness between 0.040 and 0.050 inches. Of that, only about ⅓ to ½ of the total height is actually used to provide capacitance. The rest is used to provide mechanical strength.
Implantable medical device hermetic terminals also pose another unique problem for providing substantial inductance in EMI filters. This comes from the nature of providing a hermetic seal to protect against intrusion of body fluids. A typical multi-turn inductor as described in many prior art applications (and as illustrated herein as FIG. 15) can be held loosely in one's hands. One can grasp a length of wire 36 and pass it back and forth through the center forming a multi turn inductor 34, as shown in FIG. 15. There are also a number of automatic winding device hermetic terminal, the lead wire is solidly captured at one end by the nature of the hermetic terminal (usually by a gold braze or the like). The capacitor must be mounted to the hermetic terminal in accordance with one of the many prior art references. A dilemma exists in how to make multiple turns with a bonded ferrite or a bonded ferrite slab.
Accordingly, there is a need to provide multi-element filters for implantable medical devices such that the EMI filter is designed to offer a very low attenuation in the cardiac sensing and telemetry ranges of the implantable medical device, but increase the attenuation curve very steeply above these frequencies to take into account the EMI produced by environmental emitters. Such filters should be volumetrically efficient so as to be the smallest possible size while having sufficient mechanical strength. Such filters should also be able to be hermetically sealed to protect against intrusion of body fluids into the implantable medical device. The present invention fulfills these needs and provides other related advantages.